Dell 5k not full resolution on Vega Linux

i'm having a problem with my Dell up2715k. It has a 5k resolution. I have tried Fedora 27 and Ubuntu 16.04 as well as 18.04 daily build. Neither recognizes the display with its full resolution. I'm having an amd vega. I think it has to do with display tiling not working correctly. The display only lets me only select it as dual monitor with two 2560x2880. But that unfortunately leads to fullscreen videos only have half screen size and the menu bar only on one half too. I have also tried both x11 and wayland